Change null back to string untill backend can handle None
Change null back to string untill backend can handle None

hms-dbmi/fourfront,hms-dbmi/fourfront,4dn-dcic/fourfront,4dn-dcic/fourfront,4dn-dcic/fourfront,hms-dbmi/fourfront,4dn-dcic/fourfront,hms-dbmi/fourfront,hms-dbmi/fourfront